1. Field of the Invention
The present invention is concerned with an organic light-emitting device containing a metal complex and to a method of making the same. The present invention also is concerned with new metal complexes and to new compositions containing the metal complexes for use in organic light-emitting devices.
2. Related Technology
In the last decade, much effort has been devoted to the improvement of the emission efficiency of light-emitting devices (LEDs) either by developing highly efficient materials or efficient device structures.
FIG. 1 shows a cross section of a typical LED. The device has an anode 2, a cathode 5 and a light emissive layer 4 located between the anode and the cathode. The anode may be, for example, a layer of transparent indium-tin oxide. The cathode may be, for example, LiAl. Holes and electrons that are injected into the device recombine radiatively in the light emissive layer. A further feature of the device is the optional hole transport layer 3. The hole transport layer may be a layer of polyethylene dioxythiophene (PEDOT), for example. This provides an energy level which helps the holes injected from the anode to reach the light emissive layer.
Known LED structures also may have an electron transport layer situated between the cathode 5 and the light emissive layer 4. This provides an energy level which helps the electrons injected from the cathode to reach the light emissive layer.
In an LED, the electrons and holes that are injected from the opposite electrodes are combined to form two types of excitons; spin-symmetric triplets and spin-antisymmetric singlets. Radiative decay from the singlets (fluorescence) is fast, but from the triplets (phosphorescence) it is formally forbidden by the requirement of the spin conservation.
In the past few years, many have studied the incorporation by blending of phosphorescent materials into the light emissive layer. Often, the phosphorescent material is a metal complex, however it is not so limited. Further, metal complexes also sometimes are fluorescent.
A metal complex will comprise a metal ion surrounded by ligands. A ligand in a metal complex can have several roles. The ligand can be an “emissive” ligand which accepts electrons from the metal and then emits light. Alternatively, the ligand may be present simply in order to influence the energy levels of the metal to prevent energy loss via non-radiative decay pathways (a “supporting” ligand). For example, it can be advantageous to have strong field ligands as supporting ligands coordinated to the metal to prevent energy loss via non-radiative decay pathways. Common strong field ligands are known to those skilled in this art and include CO, PPh3, and ligands where a negatively charged carbon atom bonds to the metal. N-donor ligands are also strong field ligands, although less so than those previously mentioned.
The effect of supporting ligands can be appreciated from an understanding of the mechanism by which light is emitted from a metal complex. Three reviews of luminescent metal complexes that provide an appreciation of this mechanism are referred to below.
Chem. Rev., 1987, 87,711-7434 is concerned with the luminescence properties of organometallic complexes. This review paper provides a brief summary of the excited states commonly found in organometallic complexes. The excited states that are discussed include metal-to-ligand charge-transfer (MLCT) states, which involve electronic transitions from a metal-centered orbital to a ligand-localized orbital. Thus, in a formal sense this excitation results in metal oxidation and ligand reduction. It is said that the great majority of examples of room temperature emission have been attributed to MLCT excited states.
Analytical Chemistry, Vol. 63, NO, 17, Sep. 1, 1991, 829A to 837A is concerned with the design and applications of highly luminescent transition metal complexes especially those with platinum metals (Ru, Os, Re, Rh and Ir). According to this paper, the most important design rule of luminescent transition metal complexes is that the emission always arises from the lowest excited state. Thus control of the luminescence properties of complexes hinges on control of the relative state energies and the nature and energy of the lowest excited state.
Some luminescent metal complexes having a tridentate ligand coordinated to the metal centre are known.
WO 2004/081017 relates to metal complexes with a hexadentate ligand as active components in the electronics industry.
Dalton Transactions 2005, 1, 110-115 reports the synthesis of [RhCl3tpm*], which it is said is a suitable starting material for the synthesis of heteroleptic half-sandwich complexes.
Inorganic Chemistry 2004, 43 (1), 317-323 discloses the two step synthesis of a bimetallic complex [(tmp)Ru(dppz)2dpp]4+.
Organometallics (1998), 17 (10), 1946-1955 is concerned with tuning the excited state properties of luminescent rhenium (V) benzylidyne complexes containing phosphorous and nitrogen donor ligands.
Inorganica Chimica Acta (1994), 226(1-2), 171-7 is concerned with the preparation of a series of binuclear complexes, which are derivatives of [Ru(bpy)3]2+.
Inorganic Chemistry 1993, 32(7), 1167-78 is concerned with the preparation and characterisation of Ru-based chromophore-quencher complexes.
Polyhedron (1992), 11 (16), 2119-22 is concerned with intramolecular quenching of the excited state of the tris(2,2′-bipyridyl)ruthenium II chromophore by covalently linked electron-accepting metal centres.
Journal of the American Chemical Society (1988), 110 (23), 7751-9 is concerned with ligand-substituted, mono-2,2′-bipyridene complexes of ruthenium (II).
J. Am. Chem. Soc. 1998, 120, 8747-8754 is concerned with reactions of TpRe(CO)2(THF) with aromatic molecules. This paper is not concerned with OLEDs and does not mention light emission from the Re complexes.
US 2005/0170207 discloses phosphorescent organic materials used in OLEDs. The materials are metal complexes comprising a multidentate ligand system. A metal is bound to two or more ligands and two or more of the ligands are covalently linked by one or more linking groups.
In view of the above, it will be appreciated that there is a need to identify and design new, stable metal complexes for use in LEDs which provide opportunities for improving efficiency, colour and introducing functionality.